static O[] flattenArray2(O... a) {
  new L l;
  if (a != null) for (O x : a)
    if (x instanceof O[])
      l.addAll(asList((O[]) x));
    else if (x instanceof Collection)
      l.addAll((Collection) x);
    else
      l.add(x);
  ret asObjectArray(l);
}